Fuel compositions having improved cloud point and improved storage properties

ABSTRACT

The present invention relates to a fuel composition comprising at least one biodiesel fuel and comprising 0.05 to 5% by weight of at least one polymer comprising ester groups, which comprises repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical, and repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical, and the polymer comprising ester groups has a weight-average molecular weight in the range from 5000 to 100 000 g/mol. The present invention further describes the use of polymers comprising ester groups as flow improvers in fuel compositions which comprise at least one biodiesel fuel. Surprising advantages can be achieved especially with regard to the improvement of the cloud point and the low-temperature storability.

The present invention relates to fuel compositions which comprise renewable raw materials, and to the use of polymers comprising ester groups in fuel compositions for improving the cloud point and the storage properties of fuels based on biodiesel at low temperatures.

Decreasing global mineral oil reserves and discussions regarding CO₂ imbalances resulting from the use of fossil fuels are leading to an increasing interest in alternatives based on renewable raw materials.

For instance, bioethanol is increasingly being added to commercial petroleum. In the case of diesel fuels, so-called biodiesel is used. This can either be added to diesel of fossil origin in different contents or be used in pure form. The advantage of biodiesel is the minor influence on the global CO₂ balance. For instance, the combustion of these fuels can release only as much carbon dioxide as the biomass from which it has been produced had stored. Neglecting the greenhouse gases obtained through the production of these biofuels, they do not influence CO₂ balance.

Biodiesel has the advantage that it can be obtained from a multitude of raw materials. Typical raw materials are vegetable oils (i.e. triglycerides), such as rapeseed oil, sunflower oil, soya oil, palm oil, coconut oil, coriander oil, cottonseed oil, castor oil, olive oil, peanut oil, jatropha oil, Pongamia pinnata (karanji) oil, nahar oil (Mesua ferrea L.), corn oil, almond oil, mustardseed oil, algae oil, and used vegetable oils. Further examples include oils which derive from wheat, jute, sesame, shea tree nut, arachis oil and linseed oil. It is also possible to use oils and fats of animal origin. Examples thereof are bovine tallow, pork fat, chicken fat, bone and fish oil, and further fats and oils which can be obtained in the slaughter of wild and farm animals.

The term “biodiesel” is understood in many cases to mean a mixture of fatty acid esters, usually fatty acid methyl esters (FAMEs), with chain lengths of the fatty acid component of 14 to 24 carbon atoms with 0 to 3 double bonds. The higher the number of carbon atoms and the fewer double bonds are present, the higher is the melting point of the FAME. Typical raw materials are vegetable oils (i.e. glycerides) such as rapeseed oils, sunflower oils, soya oils, palm oils, coconut oils, and in isolated cases even used vegetable oils. These are converted to the corresponding FAME by transesterification, usually with methanol under basic catalysis.

In contrast to rapeseed oil methyl ester (RME), which is widely used in Europe and typically comprises approx. 5%, occasionally even more than 6%, of C16:0+C18:0-FAME, palm oil methyl ester (PME) contains approx. 50% C16:0+C18:0-FAME. A similarly high C16:0+C18:0-FAME content is also possessed by the analogous derivatives from animal tallows, for example bovine tallow. Such a high wax content can barely be influenced by polymeric flow improvers, which are typically added at an addition rate of up to 2%. Compared to rapeseed oil, palm oil can be produced with more than three times the yield per hectare. This gives rise to immense economic advantages. However, a disadvantage is the high pour point of PME, which is about +12° C.

Polyalkyl (meth)acrylates, PA(M)As, as pour point improvers for mineral oils, either without M(M)A (e.g. U.S. Pat. No. 3,869,396 to Shell Oil Company) or with M(M)A (e.g. U.S. Pat. No. 5,312,884 to Rohm & Haas Company), or else as pour point improvers for ester-based lubricants (U.S. Pat. No. 5,696,066 to Rohm & Haas Company) have been established and described for quite a long time. Use of these polymers in fuel compositions which comprise at least one biodiesel fuel is, however, not described.

In addition, the publication WO 01/40334 (RohMax Additives GmbH) describes polyalkyl (meth)acrylates which can be used in biodiesel fuels. This publication provides a particular preparation which imparts exceptional properties to these polymers. However, there is a lack therein of examples relating to biodiesel fuels. Furthermore, the advantage of polymers which have a high proportion of particular repeat units comprising ester groups is not described.

Flow improvers based on oil-soluble polymers for mixtures of fossil diesel and biodiesel are also known (WO 94/10267, Exxon Chemical Patents Inc.). However, the examples describe only ethylene-vinyl acetate copolymers (EVAs) and copolymers which have C₁₂/C₁₄-alkyl fumarate and vinyl acetate units. There is no comprehensive and clear description of particular polymers comprising ester groups in WO 94/10267.

In addition, a series of optimized EVA copolymers for diesel/biodiesel mixtures have also become known (EP 1 541 662 to 664; WO 2008/113735 and DE 103 57 877). For instance, EP 1 541 663 describes mixtures comprising 75% by volume of diesel fuel of mineral origin and 25% by volume of biodiesel, which comprise 150 ppm of poly(dodecyl methacrylate) and 100 to 200 ppm of ethylene-vinyl acetate copolymer (EVA). However, the use of EVA is described herein as necessary. EVA is, though, quite an expensive additive. Accordingly, alternatives are desirable, in which the use of EVA can be dispensed with. There is no reference to the advantage of particular polymers comprising ester groups in EP 1 541 663.

In addition, additives for fuel mixtures which comprise mineral diesel and biodiesel are described in WO 2007/113035. In addition, the low-temperature properties achievable in diesel/biodiesel mixtures through addition of additives are not necessarily applicable to pure biodiesel fuels, since their boiling behaviour, their viscosity and hence their composition of hydrocarbons is different.

In view of the prior art, it is thus an object of the present invention to provide fuel compositions which, given a property profile which corresponds essentially to that of mineral diesel fuel, comprise a maximum proportion of renewable raw materials. At the same time, the fuel should more particularly have very good low-temperature properties. It was a further object of the present invention to provide a fuel which possesses a high stability to oxidation. In addition, the fuel should have a maximum cetane number. At the same time, the novel fuels should be producible simply and inexpensively.

It was a further object of the present invention to provide additives which are capable of lowering the cloud point of biodiesel. It was a further object of the present invention to provide fuels which, when stored below the cloud point, exhibit only minor precipitation. At the same time, this formation of precipitate should be delayed for as long as possible.

These objects and further objects which are not stated explicitly but which are immediately derivable or discernible from the connections discussed herein by way of introduction are achieved by a fuel composition having all features of claim 1. Appropriate modifications of the inventive fuel composition are protected in the dependent claims referring back to Claim 1. With regard to the use of polymers comprising ester groups as flow improvers for improving the cloud point and the low-temperature storability, Claims 17, 18 and 20 constitute a solution to the problem.

The present invention accordingly provides a fuel composition comprising at least one biodiesel fuel, characterized in that the fuel composition comprises 0.05 to 5% by weight of at least one polymer comprising ester groups, which comprises repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical, and repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical, and the polymer comprising ester groups has a weight-average molecular weight in the range from 5000 to 100 000 g/mol.

This makes it possible, in an unforeseeable manner, to provide a fuel composition which comprises at least one biodiesel fuel and which includes an excellent profile of properties. For instance, the present fuel compositions especially have a surprisingly low cloud point, a very good low-temperature storability and excellent flow properties at low temperatures.

In addition, very high proportions of palm oil alkyl esters can be used in the fuels. For ecological and economic reasons, palm oil is preferred over the typically used rapeseed oil. For instance, the yield in the production of palm oil is significantly higher than that of rapeseed oil. In addition, production of rape uses very large amounts of chemicals, especially fertilizers and crop protection compositions, which are ecologically harmful. At the same time, rape is self-incompatible in production and has to be cultured in a crop rotation system, cultivation of rape in the same field being possible only every 3 to 5 years. For this reason, a further increase in rape production is difficult.

However, palm oil alkyl esters exhibit a significantly higher cloud point (approx. +13° C. in the case of the methyl ester) compared to rapeseed oil alkyl esters; the cloud point of rapeseed oil alkyl ester is significantly lower (approx. −7° C. in the case of the methyl ester). In a particular aspect, the present invention thus enables the use of particularly high proportions of palm oil alkyl esters for producing fuel compositions, without the low-temperature properties assuming unacceptable values.

Similar advantages can also be identified with regard to other biodiesel fuels with a high proportion of saturated fatty acids. It is especially also possible to use fats of animal origin as a fuel source, which are obtainable very inexpensively.

The fuel composition of the present invention comprises at least one biodiesel fuel component. Biodiesel fuel is a substance, especially an oil, which is obtained from vegetable or animal material or both, or a derivative thereof, which can in principle be used as a replacement for mineral diesel fuel.

In a preferred embodiment, the biodiesel fuel, which is frequently also referred to as “biodiesel” or “biofuel”, comprises fatty acid alkyl esters of fatty acids having preferably 6 to 30 and more preferably 12 to 24 carbon atoms, and monohydric alcohols having 1 to 4 carbon atoms. In many cases, some of the fatty acids may contain one, two or three double bonds. The monohydric alcohols include especially methanol, ethanol, propanol and butanol, preference being given to methanol.

Examples of oils which derive from animal or vegetable material and which can be used in accordance with the invention are palm oil, rapeseed oil, coriander oil, soya oil, cottonseed oil, sunflower oil, castor oil, olive oil, peanut oil, corn oil, almond oil, palm kernel oil, coconut oil, mustardseed oil, oils derived from animal tallow, especially bovine tallow, bone oil, fish oils and used cooking oils. Further examples include oils which derive from cereals, wheat, jute, sesame, rice husks, jatropha, arachis oil and linseed oil. Surprising advantages can be achieved especially in the case of use of palm oil, soya oil, jatropha oil or animal tallow, especially beef fat, chicken fat or pork fat, as a reactant for preparing biodiesel. The fatty acid alkyl esters for use with preference can be obtained from these oils by methods known in the prior art.

Preference is given in accordance with the invention to oils with a high C16:0/C18:0-glyceride content, such as palm oils and oils derived from animal tallow, and derivatives thereof, especially the palm oil alkyl esters which are derived from monohydric alcohols. Palm oil (also: palm fat) is obtained from the flesh of the palm fruits. The fruits are sterilized and pressed. Owing to their high carotene content, fruits and oil have an orange-red colour which is removed in the refining step. The oil may contain up to 80% C18:0-glyceride.

Particularly suitable biodiesel fuels are lower alkyl esters of fatty acids. Examples here include commercial mixtures of the ethyl, propyl, butyl and especially methyl esters of fatty acids having 6 to 30, preferably 12 to 24 and more preferably 14 to 22 carbon atoms, for example of caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, margaric acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, petroselic acid, ricinoleic acid, elaeostearic acid, linoleic acid, linolenic acid, eicosanoic acid, gadoleinic acid, docosanoic acid or erucasic acid.

In a particular aspect of the present invention, a biodiesel fuel which comprises preferably at least 30% by weight, more preferably at least 35% by weight and most preferably at least 40% by weight of saturated fatty acid esters which have at least 16 carbon atoms in the fatty acid radical is used. This includes especially the esters of palmitic acid and stearic acid. Advantages which were unforeseeable for the person skilled in the art can especially be achieved with fuels which comprise at least 6% by weight, more preferably at least 10% by weight and most preferably at least 40% by weight of palmitic acid methyl ester and/or stearic acid methyl ester.

For reasons of cost, these fatty acid esters are generally used as a mixture. Biodiesel fuels usable in accordance with the invention preferably have an iodine number of at most 150, especially at most 125, more preferably at most 70 and most preferably at most 60. The iodine number is a measure known per se for the content in a fat or oil of unsaturated compounds, which can be determined to DIN 53241-1. As a result, the fuel compositions of the present invention form a particularly low level of deposits in the diesel engines. In addition, these fuel compositions exhibit particularly high cetane numbers.

In general, the fuel compositions of the present invention may comprise at least 40% by weight, especially at least 60% by weight, preferably at least 80% by weight and more preferably at least 95% by weight of biodiesel fuel.

The fuel composition of the present invention further comprises 0.05 to 5% by weight, preferably 0.08 to 3% by weight and more preferably 0.1 to 1.0% by weight of at least one polymer comprising ester groups.

Polymers comprising ester groups are understood in the present context to mean polymers which are obtainable by polymerizing monomer compositions which comprise ethylenically unsaturated compounds having at least one ester group, which are referred to hereinafter as ester monomers. Accordingly, these polymers contain ester groups as part of the side chain. These polymers include especially polyalkyl (meth)acrylates (PAMAs), polyalkyl fumarates and/or polyalkyl maleates.

Ester monomers are known per se. They include especially (meth)acrylates, maleates and fumarates, which may have different alcohol radicals. The expression “(meth)acrylates” includes methacrylates and acrylates, and mixtures of the two. These monomers are widely known. In this context, the alkyl radical may be linear, cyclic or branched. The alkyl radical may also have known substituents.

The polymers comprising ester groups contain repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical, and repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical.

The term “repeat unit” is widely known in the technical field. The present polymers comprising ester groups can preferably be obtained by means of free-radical polymerization of monomers, the ATRP, RAFT and NMP processes which will be explained later being counted among the free-radical processes in the context of the invention, without any intention that this should impose a restriction. In these processes, double bonds are opened up to form covalent bonds. Accordingly, the repeat unit is obtained from the monomers used.

The polymer comprising ester groups may contain 5 to 99.9% by weight, especially 20 to 98% by weight and more preferably 30 to 60% by weight of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical.

In a particular aspect, the polymer comprising ester groups may contain 0.1 to 90% by weight, preferably 5 to 80% by weight and more preferably 40 to 70% by weight of repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical.

In addition, the polymer comprising ester groups may contain 0.1 to 30% by weight, preferably 0.5 to 20% by weight, of repeat units derived from ester monomers having 1 to 6 carbon atoms in the alcohol radical.

The polymer comprising ester groups comprises preferably at least 40% by weight, more preferably at least 60% by weight, especially preferably at least 80% by weight and very particularly at least 95% by weight of repeat units derived from ester monomers.

Mixtures from which the inventive polymers comprising ester groups are obtainable may contain 0 to 40% by weight, especially 0.1 to 30% by weight and more preferably 0.5 to 20% by weight of one or more ethylenically unsaturated ester compounds of the formula (I)

in which R is hydrogen or methyl, R¹ is a linear or branched alkyl radical having 1 to 6 carbon atoms, R² and R³ are each independently hydrogen or a group of the formula —COOR′ in which R′ is hydrogen or an alkyl group having 1 to 6 carbon atoms.

Examples of component (I) include

(meth)acrylates, fumarates and maleates which derive from saturated alcohols, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate and pentyl (meth)acrylate, hexyl (meth)acrylate; cycloalkyl (meth)acrylates, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate; (meth)acrylates which derive from unsaturated alcohols, such as 2-propynyl (meth)acrylate, allyl (meth)acrylate and vinyl (meth)acrylate.

The compositions to be polymerized preferably contain 5 to 98% by weight, especially 20 to 90% by weight and more preferably 30 to 60% by weight of one or more ethylenically unsaturated ester compounds of the formula (II)

in which R is hydrogen or methyl, R⁴ is a linear or branched alkyl radical having 7 to 15 carbon atoms, R⁵ and R⁶ are each independently hydrogen or a group of the formula —COOR″ in which R″ is hydrogen or an alkyl group having 7 to 15 carbon atoms.

Examples of component (II) include:

(meth)acrylates, fumarates and maleates which derive from saturated alcohols, such as 2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, 2-tert-butylheptyl (meth)acrylate, octyl (meth)acrylate, 3-isopropylheptyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl (meth)acrylate, 2-methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate; (meth)acrylates which derive from unsaturated alcohols, for example oleyl (meth)acrylate; cycloalkyl (meth)acrylates such as 3-vinylcyclohexyl (meth)acrylate, bornyl (meth)acrylate; and the corresponding fumarates and maleates.

In addition, preferred monomer compositions comprise 0.1 to 90% by weight, preferably 5 to 80% by weight and more preferably 40 to 70% by weight of one or more ethylenically unsaturated ester compounds of the formula (III)

in which R is hydrogen or methyl, R⁷ is a linear or branched alkyl radical having 16 to 40 and preferably 16 to 30 carbon atoms, R⁸ and R⁹ are each independently hydrogen or a group of the formula —COOR′″ in which R′″ is hydrogen or an alkyl group having 16 to 40 and preferably 16 to 30 carbon atoms.

Examples of component (III) include (meth)acrylates which derive from saturated alcohols, such as hexadecyl (meth)acrylate, 2-methylhexadecyl (meth)acrylate, heptadecyl (meth)acrylate, 5-isopropylheptadecyl (meth)acrylate, 4-tert-butyloctadecyl (meth)acrylate, 5-ethyloctadecyl (meth)acrylate, 3-isopropyloctadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, cetyleicosyl (meth)acrylate, stearyleicosyl (meth)acrylate, docosyl (meth)acrylate and/or eicosyltetratriacontyl (meth)acrylate;

cycloalkyl (meth)acrylates such as 2,4,5-tri-t-butyl-3-vinylcyclohexyl (meth)acrylate, 2,3,4,5-tetra-t-butylcyclohexyl (meth)acrylate, and the corresponding fumarates and maleates.

The ester compounds with a long-chain alcohol radical, especially components (II) and (III), can be obtained, for example, by reacting (meth)acrylates, fumarates, maleates and/or the corresponding acids with long-chain fatty alcohols, which generally gives rise to a mixture of esters, for example (meth)acrylates with different long-chain alcohol radicals. These fatty alcohols include Oxo Alcohol® 7911, Oxo Alcohol® 7900, Oxo Alcohol® 1100; Alfol® 610, Alfol® 810, Lial® 125 and Nafol® types (Sasol); Alphanol® 79 (ICI); Epal® 610 and Epal® 810 (Afton); Linevol® 79, Linevol® 911 and Neodol® 25E (Shell); Dehydad®, Hydrenol® and Lorol® types (Cognis); Acropol® 35 and Exxal® 10 (Exxon Chemicals); Kalcol® 2465 (Kao Chemicals).

Among the ethylenically unsaturated ester compounds, the (meth)acrylates are particularly preferred over the maleates and fumarates, i.e. R2, R3, R5, R6, R8 and R9 of the formulae (I), (II) and (III) in particularly preferred embodiments are each hydrogen.

The weight ratio of units derived from ester monomers having 7 to 15 carbon atoms, preferably of the formula (II), to the units derived from ester monomers having 16 to 40 carbon atoms, preferably of the formula (III), may be within a wide range. The weight ratio of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical to repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical is preferably in the range from 5:1 to 1:30, more preferably in the range from 1:1 to 1:3, especially preferably 1.1:1 to 1:2.

Component (IV) comprises especially ethylenically unsaturated monomers which can be copolymerized with the ethylenically unsaturated ester compounds of the formulae (I), (II) and/or (III).

However, particularly suitable comonomers for polymerization according to the present invention are those which correspond to the formula:

in which R¹* and R²* are each independently selected from the group consisting of hydrogen, halogens, CN, linear or branched alkyl groups having 1 to 20, preferably 1 to 6 and more preferably 1 to 4, carbon atoms which may be substituted by 1 to (2n+1) halogen atoms, where n is the number of carbon atoms of the alkyl group (for example CF₃), α,β-unsaturated linear or branched alkenyl or alkynyl groups having 2 to 10, preferably 2 to 6 and more preferably 2 to 4, carbon atoms which may be substituted by 1 to (2n−1) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the alkyl group, for example CH₂═CCl—, cycloalkyl groups having 3 to 8 carbon atoms which may be substituted by 1 to (2n−1) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the cycloalkyl group; C(═Y*)R⁵*, C(═Y*)NR⁶*R⁷*, Y*C(═Y*)R⁵*, SOR⁵*, SO₂R⁵*, OSO₂R⁵*, NR⁸*SO₂R⁵*, PR⁵*₂, P(═Y*)R⁵*₂, Y*PR⁵*₂, Y*P(═Y*)R⁵*₂, NR⁸*₂ which may be quaternized with an additional R⁸*, aryl or heterocyclyl group, where Y* may be NR⁸*, S or O, preferably O; R⁵* is an alkyl group having from 1 to 20 carbon atoms, an alkylthio having 1 to 20 carbon atoms, OR¹⁵ (R¹⁵ is hydrogen or an alkali metal), alkoxy of 1 to 20 carbon atoms, aryloxy or heterocyclyloxy; R⁶* and R⁷* are each independently hydrogen or an alkyl group having 1 to 20 carbon atoms, or R⁶* and R⁷* together may form an alkylene group having 2 to 7, preferably 2 to 5 carbon atoms, in which case they form a 3- to 8-membered, preferably 3- to 6-membered, ring, and R⁸* is hydrogen, linear or branched alkyl or aryl groups having 1 to 20 carbon atoms; R³*l and R⁴* are independently selected from the group consisting of hydrogen, halogen (preferably fluorine or chlorine), alkyl groups having 1 to 6 carbon atoms and COOR⁹* in which R⁹* is hydrogen, an alkali metal or an alkyl group having 1 to 40 carbon atoms, or R¹* and R³* together may form a group of the formula (CH₂)_(n′) which may be substituted by 1 to 2n′ halogen atoms or C₁ to C₄ alkyl groups, or form the formula C(═O)—Y*—C(═O) where n′ is 2 to 6, preferably 3 or 4, and Y* is as defined above; and where at least 2 of the R¹*, R²*, R³* and R⁴* radicals are hydrogen or halogen.

The preferred comonomers (IV) include

hydroxyalkyl (meth)acrylates such as 3-hydroxypropyl methacrylate, 3,4-dihydroxybutyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2,5-dimethyl-1,6-hexanediol (meth)acrylate, 1,10-decanediol (meth)acrylate; aminoalkyl (meth)acrylates such as N-(3-dimethylaminopropyl)methacrylamide, 3-diethylaminopentyl methacrylate, 3-dibutylaminohexadecyl (meth)acrylate; nitriles of (meth)acrylic acid and other nitrogen-containing methacrylates, such as N-(methacryloyloxyethyl)diisobutyl ketimine, N-(methacryloyloxyethyl)dihexadecyl ketimine, methacryloylamidoacetonitrile, 2-methacryloyloxyethylmethylcyanamide, cyanomethyl methacrylate; aryl (meth)acrylates such as benzyl methacrylate or phenyl methacrylate in which the aryl radicals may each be unsubstituted or up to tetrasubstituted; carbonyl-containing methacrylates such as 2-carboxyethyl methacrylate, carboxymethyl methacrylate, oxazolidinylethyl methacrylate, N-(methacryloyloxy)formamide, acetonyl methacrylate, N-methacryloylmorpholine, N-methacryloyl-2-pyrrolidinone, N-(2-methacryloyloxyethyl)-2-pyrrolidinone, N-(3-methacryloyloxypropyl)-2-pyrrolidinone, N-(2-methacryloyloxypentadecyl)-2-pyrrolidinone, N-(3-methacryloyloxyheptadecyl)-2-pyrrolidinone; glycol dimethacrylates such as 1,4-butanediol methacrylate, 2-butoxyethyl methacrylate, 2-ethoxyethoxymethyl methacrylate, 2-ethoxyethyl methacrylate; methacrylates of ether alcohols, such as tetrahydrofurfuryl methacrylate, vinyloxyethoxyethyl methacrylate, methoxyethoxyethyl methacrylate, 1-butoxypropyl methacrylate, 1-methyl-(2-vinyloxy)ethyl methacrylate, cyclohexyloxymethyl methacrylate, methoxymethoxyethyl methacrylate, benzyloxymethyl methacrylate, furfuryl methacrylate, 2-butoxyethyl methacrylate, 2-ethoxyethoxymethyl methacrylate, 2-ethoxyethyl methacrylate, allyloxymethyl methacrylate, 1-ethoxybutyl methacrylate, methoxymethyl methacrylate, 1-ethoxyethyl methacrylate, ethoxymethyl methacrylate; methacrylates of halogenated alcohols, such as 2,3-dibromopropyl methacrylate, 4-bromophenyl methacrylate, 1,3-dichloro-2-propyl methacrylate, 2-bromoethyl methacrylate, 2-iodoethyl methacrylate, chloromethyl methacrylate; oxiranyl methacrylates such as 2,3-epoxybutyl methacrylate, 3,4-epoxybutyl methacrylate, 10,11-epoxyundecyl methacrylate, 10,11-epoxyhexadecyl methacrylate, 2,3-epoxycyclohexyl methacrylate; glycidyl methacrylate; phosphorus-, boron- and/or silicon-containing methacrylates such as 2-(dimethylphosphato)propyl methacrylate, 2-(ethylenephosphito)propyl methacrylate, dimethylphosphinomethyl methacrylate, dimethylphosphonoethyl methacrylate, diethylmethacryloyl phosphonate, dipropylmethacryloyl phosphate, 2-(dibutylphosphono)ethyl methacrylate, 2,3-butylenemethacryloylethyl borate, methyldiethoxymethacryloylethoxysilane, diethylphosphatoethyl methacrylate; vinyl halides, for example vinyl chloride, vinyl fluoride, vinylidene chloride and vinylidene fluoride; heterocyclic (meth)acrylates, such as 2-(1-imidazolyl)ethyl (meth)acrylate, 2-(4-morpholinyl)ethyl (meth)acrylate and 1-(2-methacryloyloxyethyl)-2-pyrrolidone; vinyl esters such as vinyl acetate; styrene, substituted styrenes having an alkyl substituent in the side chain, for example α-methylstyrene and α-ethylstyrene, substituted styrenes having an alkyl substituent on the ring, such as vinyltoluene and p-methylstyrene, halogenated styrenes, for example monochlorostyrenes, dichlorostyrenes, tribromostyrenes and tetrabromostyrenes; heterocyclic vinyl compounds such as 2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine, vinylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole, 2-methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactam, N-vinylbutyrolactam, vinyloxolane, vinylfuran, vinylthiophene, vinylthiolane, vinylthiazoles and hydrogenated vinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles; vinyl and isoprenyl ethers; maleic acid and maleic acid derivatives different from those mentioned under (I), (II) and (III), for example maleic anhydride, methylmaleic anhydride, maleimide, methylmaleimide; fumaric acid and fumaric acid derivatives different from those mentioned under (I), (II) and (III).

The proportion of comonomers (IV) can be varied depending on the use and property profile of the polymer. In general, this proportion may be in the range from 0 to 60% by weight, preferably from 0.01 to 20% by weight and more preferably from 0.1 to 10% by weight. Owing to the combustion properties and for ecological reasons, the proportion of the monomers which comprise aromatic groups, heteroaromatic groups, nitrogen-containing groups, phosphorus-containing groups and sulphur-containing groups can be minimized. The proportion of these monomers can therefore be restricted to 1% by weight, in particular 0.5% by weight and preferably 0.01% by weight.

The comonomers (IV) and the ester monomers of the formulae (I), (II) and (III) can each be used individually or as mixtures.

Surprising advantages can be achieved, inter alia, with polymers comprising ester groups which comprise merely a small proportion, if any, of units which are derived from hydroxyl-containing monomers. This is especially true of biodiesel fuels which have a high proportion of saturated fatty acids which have at least 16 carbon atoms in the acid radical. Accordingly, polymers comprising ester groups for use with preference in the inventive fuel mixtures preferably contain at most 5% by weight, preferably at most 3% by weight, more preferably at most 1% by weight and most preferably at most 0.1% by weight of units which are derived from hydroxyl-containing monomers. These include hydroxyalkyl (meth)acrylates and vinyl alcohols. These monomers have been detailed above.

Similarly, surprisingly good efficiency is exhibited by polymers comprising ester groups which comprise only a small proportion, if any, of repeat units which derive from monomers having oxygen-containing alcohol radicals of the formula (IV′)

where R is hydrogen or methyl, R¹⁰ is an alkyl radical which is substituted by an OH group and has 2 to 20 carbon atoms, or an alkoxylated radical of the formula (V)

in which R¹³ and R¹⁴ are each independently hydrogen or methyl, R¹⁵ is hydrogen or an alkyl radical having 1 to 20 carbon atoms, and n is an integer of 1 to 30, R¹¹ and R¹² are each independently hydrogen or a group of the formula —COOR″″ in which R″″ is hydrogen or an alkyl radical which is substituted by an OH group and has 2 to 20 carbon atoms, or an alkoxylated radical of the formula (VI)

in which R¹³ and R¹⁴ are each independently hydrogen or methyl, R¹⁵ is hydrogen or an alkyl radical having 1 to 20 carbon atoms, and n is an integer of 1 to 30.

The polymers comprising ester groups for use in accordance with the invention have a molecular weight in the range of 5000 to 100 000 g/mol, preferably in the range of 10 000 to 70 000 g/mol and more preferably in the range of 20 000 to 50 000 g/mol. These values are based on the weight-average molecular weight M_(w) of the polydisperse polymers in the composition. This parameter can be determined by GPC.

The preferred copolymers which can be obtained by polymerizing unsaturated ester compounds preferably have a polydispersity M_(w)/M_(n) in the range of 1 to 10, more preferably 1.05 to 6.0 and most preferably 1.2 to 5.0. This parameter can be determined by GPC.

The architecture of the polymers comprising ester groups is not critical for many applications and properties. Accordingly, the polymers comprising ester groups may be random copolymers, gradient copolymers, block copolymers and/or graft copolymers.

Block copolymers and gradient copolymers can be obtained, for example, by altering the monomer composition discontinuously during the chain growth. The blocks derived from ester compounds of the formulae (I), (II) and/or (III) preferably have at least 10 and more preferably at least 30 monomer units.

The preparation of the polyalkyl esters from the above-described compositions is known per se. Thus, these polymers can be obtained in particular by free-radical polymerization and related processes, for example ATRP (=Atom Transfer Radical Polymerization) or RAFT (=Reversible Addition Fragmentation Chain Transfer).

Customary free-radical polymerization is described, inter alia, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition. In general, a polymerization initiator and a chain transferrer are used for this purpose. The usable initiators include the azo initiators widely known in the technical field, such as AIBN and 1,1-azobiscyclohexanecarbonitrile, and also peroxy compounds such as methyl ethyl ketone peroxide, acetyl-acetone peroxide, dilauryl peroxide, tert-butyl per-2-ethylhexanoate, ketone peroxide, tert-butyl peroctoate, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxyisopropylcarbonate, 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, dicumyl peroxide, 1,1-bis(tert-butylperoxy)-cyclohexane, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, cumyl hydroperoxide, tert-butyl hydroperoxide, bis(4-tert-butylcyclohexyl) peroxydicarbonate, mixtures of two or more of the aforementioned compounds with one another, and mixtures of the aforementioned compounds with compounds which have not been mentioned but can likewise form free radicals. Suitable chain transferrers are in particular oil-soluble mercaptans, for example n-dodecyl mercaptan or 2-mercaptoethanol, or else chain transferrers from the class of the terpenes, for example terpinolene.

The ATRP process is known per se. It is assumed that it is a “living” free-radical polymerization, without any intention that the description of the mechanism should impose a restriction. In these processes, a transition metal compound is reacted with a compound which has a transferable atom group. This transfers the transferable atom group to the transition metal compound, which oxidizes the metal. This reaction forms a radical which adds onto ethylenic groups. However, the transfer of the atom group to the transition metal compound is reversible, so that the atom group is transferred back to the growing polymer chain, which forms a controlled polymerization system. The structure of the polymer, the molecular weight and the molecular weight distribution can be controlled correspondingly.

This reaction is described, for example, by J-S. Wang, et al., J. Am. Chem. Soc., vol. 117, p. 5614-5615 (1995), by Matyjaszewski, Macromolecules, vol. 28, p. 7901-7910 (1995). In addition, the patent applications WO 96/30421, WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclose variants of the ATRP explained above.

In addition, the inventive polymers may be obtained, for example, also via RAFT methods. This process is presented in detail, for example, in WO 98/01478 and WO 2004/083169, to which reference is made explicitly for the purposes of disclosure.

In addition, the inventive polymers are obtainable by NMP processes (nitroxide-mediated polymerization), which are described, inter alia, in U.S. Pat. No. 4,581,429.

These methods are described comprehensively, in particular with further references, inter alia, in K. Matyjaszewski, T. P. Davis, Handbook of Radical Polymerization, Wiley Interscience, Hoboken 2002, to which reference is made explicitly for the purposes of disclosure.

The polymerization may be carried out at standard pressure, reduced pressure or elevated pressure. The polymerization temperature too is uncritical. However, it is generally in the range of −20°-200° C., preferably 0°-130° C. and more preferably 60°-120° C.

The polymerization may be carried out with or without solvent. The term solvent is to be understood here in a broad sense.

The polymerization is preferably carried out in a nonpolar solvent. These include hydrocarbon solvents, for example aromatic solvents such as toluene, benzene and xylene, saturated hydrocarbons, for example cyclohexane, heptane, octane, nonane, decane, dodecane, which may also be present in branched form. These solvents may be used individually and as a mixture. Particularly preferred solvents are mineral oils, diesel fuels of mineral origin, natural vegetable and animal oils, biodiesel fuels and synthetic oils (e.g. ester oils such as dinonyl adipate), and also mixtures thereof. Among these, very particular preference is given to mineral oils and mineral diesel fuels.

The inventive fuel composition may comprise further additives in order to achieve specific solutions to problems. These additives include dispersants, for example wax dispersants and dispersants for polar substances, demulsifiers, defoamers, lubricity additives, antioxidants, cetane number improvers, detergents, dyes, corrosion inhibitors and/or odorants.

For example, the inventive fuel composition may comprise ethylene copolymers which are described, for example, in EP-A-1 541 663. These ethylene copolymers may contain 8 to 21 mol % of one or more vinyl and/or (meth)acrylic esters and 79 to 92% by weight of ethylene. Particular preference is given to ethylene copolymers containing 10 to 18 mol % and especially 12 to 16 mol % of at least one vinyl ester. Suitable vinyl esters derive from fatty acids having linear or branched alkyl groups having 1 to 30 carbon atoms. Examples include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl hexanoate, vinyl heptanoate, vinyl octanoate, vinyl laurate and vinyl stearate, and also esters of vinyl alcohol based on branched fatty acids, such as vinyl isobutyrate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl isononanoate, vinyl neononanoate, vinyl neodecanoate and vinyl neoundecanoate. Comonomers which are likewise suitable are esters of acrylic acid and methacrylic acid having 1 to 20 carbon atoms in the alkyl radical, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n- and isobutyl (meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, tetradecyl (meth)acrylate, hexadecyl (meth)acrylate, octadecyl (meth)acrylate, and also mixtures of two, three or four or else more of these comonomers.

Particularly preferred terpolymers of vinyl 2-ethylhexanoate, of vinyl neononanoate and of vinyl neodecanoate contain, apart from ethylene, preferably 3.5 to 20 mol %, in particular 8 to 15 mol %, of vinyl acetate and 0.1 to 12 mol %, in particular 0.2 to 5 mol %, of the particular long-chain vinyl ester, the total comonomer content being between 8 and 21 mol %, preferably between 12 and 18 mol %. Further preferred copolymers contain, in addition to ethylene and 8 to 18 mol % of vinyl esters, also 0.5 to 10 mol % of olefins such as propene, butene, isobutylene, hexene, 4-methylpentene, octene, diisobutylene and/or norbornene.

The ethylene copolymers preferably have molecular weights which correspond to melt viscosities at 140° C. of from 20 to 10 000 mPas, in particular 30 to 5000 mPas and especially 50 to 1000 mPas. The degrees of branching determined by means of ¹H NMR spectroscopy are preferably between 1 and 9 CH₃/100 CH₂ groups, in particular between 2 and 6 CH₃/100 CH₂ groups, for example 2.5 to 5 CH₃/100 CH₂ groups, which do not stem from the comonomers.

Such ethylene copolymers are described in detail, inter alia, in DE-A-34 43 475, EP-B-0 203 554, EP-B-0 254 284, EP-B-0 405 270, EP-B-0 463 518, EP-B-0 493 769, EP-0 778 875, DE-A-196 20 118, DE-A-196 20 119 and EP-A-0 926 168.

Preference is given in this context to ethylene-vinyl acetate copolymers and terpolymers which, in addition to ethylene and vinyl acetate repeat units, also have repeat (meth)acrylic ester units. These polymers may be structured, for example, as random copolymers, as block copolymers or as graft copolymers.

In a preferred embodiment, the inventive fuel composition may comprise 0.0005 to 2% by weight, preferably 0.01 to 0.5% by weight, of ethylene copolymers.

For reasons of cost, however, a proportion of the above-described ethylene copolymers can be dispensed with in a further embodiment, in which case these fuel compositions without a significant proportion of ethylene copolymers have outstanding properties. In this specific embodiment, the proportion of ethylene copolymers may preferably be at most 0.05% by weight, more preferably at most 0.001% by weight and most preferably at most 0.0001% by weight.

In addition to the components described above, a fuel composition may comprise further components. These include especially diesel fuels of mineral origin. For reasons of environmental protection, the proportion of diesel fuels of mineral origin may preferably be limited to at most 60% by weight, more preferably at most 40% by weight and most preferably at most 15% by weight.

The inventive fuel compositions have outstanding low-temperature properties. Accordingly, the use of polymers which comprise ester groups and comprise repeat units derived from unsaturated esters having 7 to 15 carbon atoms in the alcohol radical, and repeat units derived from unsaturated esters having 16 to 40 carbon atoms in the alcohol radical, in a concentration of 0.05 to 5% by weight, as flow improvers in fuel compositions which comprise at least one biodiesel fuel, constitutes a further aspect of the present invention.

More particularly, the pour point (PP) to ASTM D97 preferably has values less than or equal to 12° C., preferably less than or equal to 10° C. and more preferably less than or equal to 0° C. Based on the biodiesel fuel without addition of polymers for use in accordance with the invention, it is unexpectedly possible to achieve improvements in the pour point of 1° C., preferably 3° C. and most preferably 5° C.

The limit of the cold filter plugging point (CFPP) measured to DIN EN 116 is preferably at most 12° C., more preferably at most 10° C. and more preferably at most 0° C. In addition, the cloud point (CP) to ASTM D2500 of preferred fuel compositions may assume values less than or equal to 12° C., preferably less than or equal to 10° C. and more preferably less than or equal to 0° C.

Surprisingly, it is especially possible to reduce the cloud point with the polymers comprising ester groups for use in accordance with the invention. This reduction is possible even in the case of biodiesel fuels with a particularly high proportion of long-chain saturated fatty acid units. This finding is surprising especially because customary flow improvers, especially polymers which have, for example, ethylene-vinyl acetate (EVA) units, which have been developed to influence the CFPP, can cocrystallize with the paraffins in the case of fossil diesel or the fatty acid alkyl esters as the additized fuel is cooled. This prevents further agglomeration of the individual crystals and ensures the filterability of the fuel at relatively low temperatures. The formation of the primary crystals and the temperature at which they form are, however, not influenced by such additives. Since the cloud point is by definition that point at which crystallization sets in, it is obvious that the cloud point is barely influenced by such additives. The present invention therefore also provides for the use of polymers which comprise ester groups and comprise repeat units derived from unsaturated esters having 7 to 15 carbon atoms in the alcohol radical, and repeat units derived from unsaturated esters having 16 to 40 carbon atoms in the alcohol radical, in a concentration of 0.05 to 5% by weight, for improving the cloud point of fuel compositions which comprise at least one biodiesel fuel.

In this case, the cloud point can be reduced by at least 1° C., preferably by at least 2° C. or by at least 3° C. and most preferably by at least 5° C. These figures are based on the cloud point of the biodiesel fuel without the addition of polymers comprising ester groups for use in accordance with the invention.

In the case of a surprisingly low use of up to 0.6% by weight of polymers comprising ester groups, it is in many cases possible to achieve improvements in the cloud point by at least 1° C., preferably by at least 3° C.

A further surprising aspect of the present fuel composition is its outstanding low-temperature storability. Accordingly, the inventive fuel compositions can be stored even at temperatures below the cloud point without this being accompanied by any significant separation of the fuel or significant formation of precipitate. This aspect is essential especially in the case of brief occurrence of unexpectedly low temperatures.

The cetane number to DIN 51773 of inventive fuel compositions is preferably at least 50, more preferably at least 53, especially at least 55 and most preferably at least 58.

The viscosity of the present fuel compositions may lie within a wide range, which may be adjusted to the intended use. This adjustment can be effected, for example, through selection of the biodiesel fuels. In addition, the viscosity can be varied by the amount and the molecular weight of the polymers comprising ester groups used. The kinematic viscosity of preferred fuel compositions of the present invention is in the range from 1 to 10 mm²/s, more preferably 2 to 5 mm²/s and especially preferably 2.5 to 4 mm²/s, measured at 40° C. to ASTM D445.

The present invention will be illustrated hereinafter with reference to examples and comparative examples, without any intention that this should impose a restriction.

EXAMPLES AND COMPARATIVE EXAMPLES General Method for Preparing the Polymers

600 g of monomer composition according to the composition detailed in each case in Table 1 and n-dodecyl mercaptan (20 g to 2 g depending on the desired molecular weight) are mixed. 44.4 g of this monomer/regulator mixture are charged together with 400 g of carrier oil (e.g. 100N mineral oil, synthetic dinonyl adipate or vegetable oil) into the 2 l reaction flask of an apparatus with sabre stirrer, condenser, thermometer, feed pump and N₂ feed line. The apparatus is inertized and heated to 100° C. with the aid of an oil bath. The remaining amount of 555.6 g of monomer/regulator mixture is admixed with 1.4 g of tert-butyl peroctoate. Once the mixture in the reaction flask has attained a temperature of 100° C., 0.25 g of tert-butyl peroctoate is added, and the feed of the monomer/regulator/initiator mixture by means of a pump is started simultaneously. The addition is effected uniformly over a period of 210 min at 100° C. 2 h after the end of feeding, another 1.2 g of tert-butyl peroctoate are added and the mixture is stirred at 100° C. for a further 2 h. A 60% clear concentrate is obtained.

The mass-average molecular weight M_(w) and the polydispersity index PDI of the polymers were determined by GPC. The measurements were effected in tetrahydrofuran at 35° C. against a polymethyl methacrylate calibration curve composed of a set of ≧25 standards (Polymer Standards Service or Polymer Laboratories), whose M_(peak) was distributed in a logarithmically uniform manner over the range of 5×10⁶ to 2×10² g/mol. A combination of six columns (Polymer Standards Service SDV 100 Å/2x SDV LXL/2x SDV 100 Å/Shodex KF-800D) was used. To record the signal, an RI detector (Agilent 1100 Series) was used.

TABLE 1 Properties of the polymers used Monomer composition Mw PDI Polymer (weight ratio) [g/mol] (Mw/Mn) Example 1 SMA-LMA 98 300 2.25 60-40 Example 2 SMA-LMA 39 400 2.12 60-40 Example 3 SMA-LMA-BhMA-IDMA 44 000 2.25 40-40-10-10 Comparative DPMA-HEMA 22 000 1.93 Example 1 90-10 Comparative DPMA 65 600 2.08 Example 2 100 SMA: alkyl methacrylate which has 16 to 18 carbon atoms in the alkyl radical LMA: alkyl methacrylate which has about 10 carbon atoms in the alkyl radical, the alkyl radical being predominantly linear BhMA: alkyl methacrylate which has about 22 carbon atoms in the alkyl radical IDMA: alkyl methacrylate which has about 10 carbon atoms in the alkyl radical, the alkyl radical being predominantly branched DPMA: alkyl methacrylate which has 12 to 15 carbon atoms in the alkyl radical HEMA: 2-hydroxyethyl methacrylate

Subsequently, the polymers thus obtained were studied in various biodiesel compositions. To this end, more particularly, a fatty acid methyl ester formed from jatropha oil of Indian origin with a proportion of palmitic acid methyl ester of 15.0% by weight and a proportion of stearic acid methyl ester of 6.8% by weight (JME), a fatty acid methyl ester formed from soya oil of North American origin with a proportion of palmitic acid methyl ester of 10.7% by weight and a proportion of stearic acid methyl ester of 4.1% by weight (SME), and a fatty acid methyl ester formed from palm oil of Malaysian origin with a proportion of palmitic acid methyl ester of 43.7% by weight and a proportion of stearic acid methyl ester of 4.4% by weight (PME), were used. The amount of polymer used was in each case 600 ppm.

To study the low-temperature properties, the cloud point (CP) to ASTM D2500 of the fuel compositions or the low-temperature storability was determined.

In order to evaluate the influence of additives on the storage of FAME, samples with and without additives were stored at temperatures below the cloud point for 72 hours (cold storage test, CST). This was done in a Julabo cryostat. After 24 and 72 hours, the samples were assessed visually. The samples were rated on a scale of 1-10. A methyl ester from soya oil (SME) of North American origin and a methyl ester from palm oil (PME) of Malaysian origin were used.

TABLE 2 Assessment of the biodiesel samples after storage 1 sample has solidified completely 2 some liquid above a solid block 3 significant amount (25%-50%) of liquid above a solid layer 4 majority of the sample liquefied (50%+) over a solid layer 5 liquid layer but significant deposited solid layer 6 liquid layer but small deposited solid layer 7 liquid layer over a small amount of sediment 8 sample liquid but slightly cloudy with a small amount of solid sediment 9 sample completely liquid but cloudy 10 sample completely liquid and clear

The results obtained are shown in Table 3, 4 or Table 4.

TABLE 3 Cloud point (CP) of fuel compositions Proportion of the Cloud point (CP) polymer in the Cloud point (CP) to ASTM D2500 mixture to ASTM D2500 of PME Polymer used [% by wt.] of JME [° C.] [° C.] unadditized — 5 14 Example 1 0.6 1 11 Example 2 0.6 2 12 Example 3 0.6 1 11 Comparative 0.6 3 14 Example 1 Comparative 0.6 3 14 Example 2

TABLE 4 Storage of SME at −5° C. Proportion of Cold storage Cold storage the polymer in test (CST) test (CST) the mixture assessment assessment Polymer used [% by wt.] after 24 h after 72 h unadditized — 3 3 Example 1 0.6 8 8 Example 2 0.6 9 9 Example 3 0.6 8 8 Comparative Example 1 0.6 3 3 Comparative Example 2 0.6 3 3

TABLE 5 Storage of PME at 10° C. Proportion of Cold storage Cold storage the polymer in test (CST) test (CST) the mixture assessment assessment Polymer used [% by wt.] after 24 h after 72 h unadditized — 1 1 Example 1 0.6 6 5 Example 2 0.6 7 6 Example 3 0.6 6 5 Comparative Example 1 0.6 1 1 Comparative Example 2 0.6 1 1

The data shown above show clearly that the low-temperature storability of fuel compositions with biodiesel components can surprisingly be increased by the addition of polymers comprising ester groups. 

1. A fuel composition comprising at least one biodiesel fuel, wherein the fuel composition further comprises 0.05 to 5% by weight of at least one polymer comprising ester groups, which comprises repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical, and repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical, and the polymer comprising ester groups has a weight-average molecular weight in the range from 5000 to 100 000 g/mol.
 2. The fuel composition according to claim 1, wherein the fuel composition comprises at least 80% by weight of biodiesel fuel.
 3. The fuel composition according to claim 1, wherein the polymer comprising ester groups is selected from the group consisting of a polyalkyl (meth)acrylate (PAMA), a polyalkyl fumarate and a polyalkyl maleate.
 4. The fuel composition according to claim 2, wherein the polymer comprising ester groups comprises 40 to 70% by weight of units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical.
 5. The fuel composition according to claim 2, wherein the polymer comprising ester groups comprises 30 to 60% by weight of units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical.
 6. The fuel composition according to t claim 2, wherein the polymer comprising ester groups is obtainable by polymerizing a monomer mixture which comprises 0 to 40% by weight of one or more ethylenically unsaturated ester compounds of the formula (I)

in which R is hydrogen or methyl, R¹ is a linear or branched alkyl radical having 1 to 6 carbon atoms, R² and R³ are each independently hydrogen or a group of the formula —COOR′ in which R′ is hydrogen or an alkyl group having 1 to 6 carbon atoms, 10 to 98% by weight of one or more ethylenically unsaturated ester compounds of the formula (II)

in which R is hydrogen or methyl, R⁴ is a linear or branched alkyl radical having 7 to 15 carbon atoms, R⁵ and R⁶ are each independently hydrogen or a group of the formula —COOR″ in which R″ is hydrogen or an alkyl group having 7 to 15 carbon atoms, and 0.1 to 80% by weight of one or more ethylenically unsaturated ester compounds of the formula (III)

in which R is hydrogen or methyl, R⁷ is a linear or branched alkyl radical having 16 to 40 carbon atoms, R⁸ and R⁹ are each independently hydrogen or a group of the formula —COOR′″ in which R′″ is hydrogen or an alkyl group having 16 to 40 carbon atoms.
 7. The fuel composition according to claim 6, wherein the weight ratio of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical to repeat units derived from ester monomers having 16 to 40 carbon atoms in the alcohol radical is in the range from 1:1 to 1:3.
 8. The fuel composition according to claim 2, wherein the biodiesel fuel comprises fatty acid esters derived from monohydric alcohols having 1 to 4 carbon atoms.
 9. The fuel composition according to claim 8, wherein the monoester is a methyl ester.
 10. The fuel composition according to claim 9, wherein the fuel comprises at least 6% by weight of palmitic acid methyl ester and/or stearic acid methyl ester.
 11. The fuel composition according to claim 2, wherein the biodiesel fuel comprises at least 35% by weight of saturated fatty acid esters which have at least 16 carbon atoms in the fatty acid radical.
 12. The fuel composition according to claim 2, wherein the biodiesel fuel is derived from palm oil, soya oil, jatropha oil or animal tallow.
 13. The fuel composition according to claim 2, wherein the fuel composition comprises at least one additive.
 14. The fuel composition according to claim 12, wherein at least one additive is selected from the group consisting of a dispersant, a demulsifier, a defoamer, a lubricity additive, an antioxidant, a cetane number improver, a detergent, a dye, a corrosion inhibitor and an odorant.
 15. The fuel composition according to claim 2, wherein the fuel composition comprises 0.1 to 1% by weight of at least one polymer comprising ester groups.
 16. The fuel composition according to claim 2, wherein the fuel composition comprises at most 0.05% by weight of ethylene copolymer.
 17. (canceled)
 18. The fuel composition which comprises at least one biodiesel fuel according to claim 1 wherein the cloud point of the fuel composition decreases from a value less than or equal to 12° C. by at least 1° C.
 19. (canceled)
 20. The fuel composition which comprises at least one biodiesel fuel according to claim 1 wherein the fuel composition may be stored at temperatures below the cloud point without significant separation of the fuel or formation of precipitate.
 21. The fuel composition according to claim 12, wherein the animal tallow is selected from a group consisting of beef fat, chicken fat, and pork fat.
 22. The fuel composition according to claim 1, wherein the fuel composition has a pour point having a value less than or equal to 12° C. 